In an optical communication system, it is necessary to provide a light source which is spectrally narrow and stable in frequency. It is further necessary to modulate the light beam without adversely affecting the source frequency or spectral line width.
Solid state laser diode oscillators are frequently used as the source of a light beam for optical fiber systems. The output of a laser diode oscillator may be stabilized by an external resonant cavity. It is known to modulate laser diode oscillators by turning the laser diode on and off. Such modulation tends to perturb both the frequency and the line width of the laser diode output. Although the modulation inherently produces a broadening of the laser diode output, the modulation produces perturbations which are well in excess of that expected by a Fourier transformed analysis of the modulation signal. Thus, a need has arisen for improved optical modulation techniques.
In any optical communication system, reflection of power back into a laser source, particularly a laser diode oscillator, alters the oscillator frequency appreciably, and variable reflection creates frequency instability. For this reason, unidirectional devices called optical isolators are used to prevent light power from returning to the laser source. Prior art optical isolators have included two rotated polarizers separated by means for rotating the plane of polarization with a magnetic field. The plane of polarization for a forward light beam is rotated to pass through both polarizers, while the plane of polarization of a beam passing in the reverse direction is rotated in such a way that the beam is blocked. These devices require a polarized light beam for proper operation and are relatively expensive.
It is a general object of the present invention to provide an improved optical modulator.
It is another object of the present invention to provide an improved optical isolator.
It is a further object of the present invention to provide a modulated optical source including a light source and a separate optical modulator.
It is a further object of the present invention to provide a laser diode modulator in which oscillations are suppressed.
It is yet another object of the present invention to provide an optical isolator for unidirectional optical transmission which is independent of polarization.
It is still another object of the present invention to provide a modulated optical source including an isolation feature which prevents reverse light beams from reaching the optical source.
It is yet another object of the present invention to provide an optical modulator which is relatively low in cost.
It is yet another object of the present invention to provide a modulated optical source wherein the modulation does not appreciably affect the frequency or spectral line width of the source.